1. Field of the Invention
This invention relates to holographic scanners in general, and specifically to a holographic scanner which is relatively insensitive to small variations in wavelength.
2. Description of the Prior Art
Scanners may be used to either write to a media, or read from a media. When writing to a media, information is imposed on the beam prior to the creation of successive sweeps of the beam. Scanners may also be used for reading, in which case the beam impinging on the target has a constant intensity and the light reflected or transmitted by the target being read is monitored to create a signal representative of the information on the scanned target.
Rotating scanners, called polygons, include a device having a plurality of planar mirrors disposed uniformly around a cylinder. The planes of the mirrors are parallel to the axis of the cylinder, and the radii through the centers of the mirrors are at uniform angular spacing around the axis. A motor rotates the device at a high speed and a stationary beam of light is directed at the device which reflects the incident beam and creates successive sweeps of the reflected light beam across an image plane, each along the same linear path.
Holographic scanners use a diffractive surface instead of a reflective surface for deflecting the beam. Such diffractive devices may include a disc of light-transmissive or reflective material, called a holographic deflector having a plurality of identical facets, each of which contains a diffraction grating. The lines of the grating on the holographic deflector may be perpendicular to a radius bisecting the facet, or they may be parallel to such a radius. The two types of gratings are called tangential and radial, respectively.
Holographic laser scanners are known to be relatively insensitive to a wobble of the deflector substrate because they utilize the physical process of diffraction to deflect a laser beam rather than using reflection from a mirrored surface, as is the case with polygon type laser scanners. The mirrored surfaces in polygon scanners double angular errors, such as deflector wobble, and facet to facet perpendicularity errors. Holographic deflectors, on the other hand, reduce the magnitude of angular deflection in the output beam. These holographic deflectors typically use planar gratings to scan the laser beam across the image plane.
Prior art holographic scanner rely on the use of wavelength stabilized laser sources to maintain high precision spot placement while scanning. This is due to the fact that a high spatial frequency grating is used to deflect the laser with a grating pitch which is typically 0.51 to 2.01 .mu.m. These gratings are wavelength dispersive, and small variations in wavelength cause substantial variations in the output beam's deflection angle. In the page direction a variation in laser source wavelength causes a vertical displacement of the scan line, while in the scan direction, the spot location is directly dependent on the laser wavelength as well as the deflector rotation angle. Laser wavelength changes cause horizontal spot position errors, which increase as a function of deflected scan angle. Thus the farther the spot is from the center of scan, the larger the error.
Semiconductor lasers used in light scanning apparatus, or raster output scanners, provide a compact and low cost light source and a capability for high speed direct modulation of the laser. However, when the semiconductor lasers are current modulated for information processing, their emission wavelength changes depending on the laser chip temperature and the operating current. In prior art holographic scanners using a semiconductor laser, the position of the scanning beam varies as a function of the light source wavelength, and changes both in the scan direction and in the cross scan or page direction, producing artifacts.
U.S. Pat. No. 4,753,503 discloses a laser scanning system using a rotating linear diffraction grating and a semiconductor laser light source. This patent teaches a method of minimizing the beam position error due to wavelength changes by using a stationary compensating grating. The compensating grating is placed parallel to the rotating grating and has a grating spatial frequency identical to the rotating grating. (Identical spatial frequency means the pitch of the grating on the stationary device is the same as the pitch of the grating on the holographic deflector.) This arrangement provides complete correction of beam position error at the center of scan, partial correction toward the end of the scan in the cross scan or page direction, but no correction for the scan line length changes.
U.S. Pat. No. 4,810,046 describes a light beam scanning apparatus using a linear grating rotating hologram with a stationary post hologram to compensate for beam position shift due to the wavelength shift of the semiconductor light source. This scheme reduces the position error in the cross scan direction to within plus or minus 2 .mu.m for plus or minus 0.3 nm shift in wavelength compared to an uncompensated error of plus or minus 900 .mu.m. In the line scan direction the position error is reduced to 5 .mu.m for a 0.3 nm wavelength shift.
U.S. Pat. No. 4,505,537 describes a light scanning apparatus which uses a linear rotating grating and spherical and cylindrical refractive optics to minimize beam position error caused by changes in wavelength of the light source. The optical system is designed to maintain the scanning surface to be optically conjugated by using geometric optics with the grating surface. This scheme reduces the beam position error to 5 .mu.ms for 0.3 nm in the page direction due to a shift in wavelength, but provides no correction in the scan line direction.
U.S. Pat. No. 5,182,659 describes a light scanning system which uses a pre-scan and post-scan holograms along with a multifaceted rotating grating. This arrangement has the same limited performance as the previously mentioned patents.